Fast LPS-induced IgM response of B-1 and MZ cells. (A) FACS-purified peritoneal B-1a, B-1b, and B-2 cells, and splenic B-1a, MZ, and B-2 cells of induced B-Indu-Rag1^fl/fl mice or control-treated wild-type mice (BALB/c and B-Indu-Rag1^fl/wt mice) were cultured in the absence or presence of LPS. For each condition, the number of antibody-secreting cells per 10⁵ cells was determined by IgM-specific ELISPOT at indicated time points. For details on sorting strategies and purities, see supplemental Figure 4. (B) Peritoneal cells from 3 pooled Rag1^−/− recipient mice 4 weeks after transfer were stained for IgM^a, IgM^b, CD43, CD19, and CD5, and analyzed by flow cytometry. Numbers in quadrants indicate percentages of cells per viable, lymphoid cells. (C) Peritoneal washout cells from Rag1^−/− recipient mice were cultured in the absence or presence of LPS. The number of antibody-secreting cells per 10⁶ cells was determined by an IgM allotype-specific ELISPOT at indicated time points and normalized with regard to the allotype-positive input at day 0.